Compressed air foam (CAF) was developed in the 1970s in Texas as an innovative approach for fighting grassland fires in areas where water is extremely scarce. The system combines two technologies, an agent to reduce the surface tension of water and compressed air to produce an expanded volume of fire extinguishing agent. The surface tension reduction, which makes water much more efficient as an extinguishing agent, is accomplished by introducing a small percentage of Class A foam concentrate into the water stream. Compressed air is then injected into the solution to expand the foam, creating a mass of foam bubbles to provide a much greater volume of extinguishing agent in a form that has the ability to stick to vertical surfaces and flow over horizontal surfaces, forming an insulating layer. The foam bubbles are more efficient at absorbing heat than plain water, whether it is in the form of a solid stream or small droplets. CAF can be discharged from both handlines and master stream devices.
There are two main types of nozzle used to disperse the CAF, namely aspirated nozzles or compressed air nozzles. A compressed air nozzle generally operates as follows: Primary mixing of the foam components occurs in a mixing chamber. The compressed air nozzle allows the injection of pressurized gas or air at a point just beyond the mixing chamber in an aftermix chamber. The injected pressurized gas in the aftermix chamber provides additional mixing of the foam and also propels the foam, resulting in an improved spray pattern. The “fineness” of the spray pattern can be altered by adjusting the amount of injected pressurized gas into the aftermix chamber. The injected pressurized gas also flushes the foam during and after every use from the aftermix chamber and the nozzle attached to the aftermix chamber, thus, eliminating the need for replacing or cleaning the nozzle after each use.
Nozzles of this type, due mainly to the use of the injected pressurised gas to both convey and expand the foam, suffer from a limited degree to which the foam can be expanded. In addition to this, the foam may have time to collapse between injection of the pressurised gas and dispersion from the nozzle.
An aspirated nozzle or eductor nozzle generally operates on a venturi basis in which, as the foam flows through the nozzle, air is drawn from outside the nozzle and injected into the foam flow. In operation, the pressure energy of the motive liquid is converted to velocity energy by the converging nozzle. The high velocity liquid flow then entrains the suction liquid. This device has a disadvantage in that the amount of air which is drawn into the nozzle is directly proportional to the flow rate of the foam through the nozzle. This limits the degree to which the foam can be expanded.
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